Many gardening terms are not at all obvious, creating a false sense of difficulty or complexity. My ‘In the know’ guides are designed to introduce you to time-honoured techniques in easy to understand terms so that we can all benefit from them.
What is it?
‘Hardening off’ is a process by which plants are gradually accustomed to a change of growing conditions, particularly if they are moving from somewhere warm and sheltered, like a greenhouse, to somewhere cool and exposed, like the garden. The process might just as easily be called ‘toughening up’ as the result is a plant that’s stronger, more resilient and able to brave the elements thanks to thicker stems, tougher leaves and a vigorous root system.
Why do it?
By and large, plants do not like dramatic fluctuations in temperature or degrees of exposure. Like humans, they can be averse to change with a tendency to sulk for a while when forced into a new situation.
If a plant is moved quickly from the comfort and protection of a windowsill or greenhouse into a cold, wet, windy garden then its growth can be ‘checked’ i.e. it will stop developing until it has acclimatised to its new home. In effect the plant goes into shock and in extreme cases it might even curl up and die. The main impact of ‘checking’ is that plants will reach their peak later and that might reduce the amount of time they have to flower or fruit. While they are sulking and not growing, plants are also weaker and more vulnerable to pests and diseases. A rapidly growing plant is a healthy and happy plant!
Keep in mind that plants purchased from a nursery, garden centre or mail-order company may well have spent their previous life in a greenhouse so they could be equally vulnerable and in need of hardening off when they reach your home.
When to do it
Hardening off typically takes place from late April through to early June, depending on whereabouts in the country you live and what the weather is like. The aim is to have plants ready to be planted outdoors as soon as the risk of frost has past - for many of us that’s late May. For large, hardy plants, i.e. those that have some frost tolerance, the process of hardening off can take as little as a week. For half-hardy plants, in can take two to three weeks, depending on the weather conditions. Before hardening off, plants should be almost at the size you’d expect to plant them out at - so not too small. I would personally not begin until daytime temperatures are regularly 10ºC or more.
How to do it
Gently, slowly and in small steps is the way to go. If your plants have been living on an indoor windowsill, step one would be to move them into an unheated greenhouse or perhaps a well-lit shed during the daytime for a few hours each day, returning them to the windowsill overnight. The next step would be to leave them in the greenhouse or shed all day and all night. Next you might put them outside in a sheltered position for a few hours every day, returning them to the shed or greenhouse when night falls. Finally, you might leave them outside all day with just a covering of fleece after dark, remembering to protect them from slugs and snails of course!
There are no hard and fast rules as it depends what facilities you have. If you don’t have anything other than a windowsill and a garden or balcony, simply place your plants outside in a sheltered spot for an increasing number of hours each day, starting with fine days but avoiding windy ones. A combination of wind and bright sunshine can quickly dehydrate young plants causing wilting, which will itself ‘check’ the plant’s growth. A warm, sheltered, shaded spot is perfect to begin with.
You don’t have to stick religiously to a timetable - watch the weather, use your common sense and just keep in mind that the whole aim is introduce your plants to their new quarters as gently and imperceptibly as possible. If you do forget about them one night, just continue the process, although nothing will resurrect half-hardy plants if they are caught by the frost.